236 ISLAND LIFE 



far more extensive period which includes all possibility of 

 life upon the earth. 



Concluding Remarks. — In the present chapter I have 

 endeavoured to show that, combining the measured rate of 

 denudation with the estimated thickness and probable 

 extent of the known series of sedimentary rocks, we may 

 arrive at a rude estimate of the time occupied in the for- 

 mation of those rocks. From another point of departure — 

 that of the probable date of the Miocene period, as deter- 

 mined by the epoch of high excentricity supposed to have 

 aided in the production of the Alpine glaciation during 

 that period, and taking the estimate of geologists as to the 

 proportionate amount of change in the animal world since 

 that epoch — we obtain another estimate of the duration of 

 geological time, which, though founded on far less secure 

 data, agrees pretty nearly with the former estimate. The 

 time thus arrived at is immensely less than the usual 

 estimates of geologists, and is so far within the limits of 

 the duration of the earth as calculated by Sir William 

 Thomson, as to allow for the development of the lower 

 organisms an amount of time anterior to the Cambrian 

 period several times greater than has elapsed between that 

 period and the present day. I have further shown that, in 

 the continued mutations of climate produced by high 

 excentricity and opposite phases of precession, even though 

 these did not lead to glacial epochs, we have a motive 

 power well calculated to produce far more rapid organic 

 changes than have hitherto been thought possible ; 

 while in the enormous amount of specific variation (as 

 demonstrated in an earlier chapter), we have ample 

 material for that power to act upon, so as to keep the 

 organic world in a state of rapid change and development 

 proportioned to the comparatively rapid changes in the 

 earth's surface. 



We have now finished the series of preliminary studies 

 of the biological conditions and physical changes which 

 have affected the modification and dispersal of organisms, 

 and have thus brought about their actual distribution on 



